We have previously demonstrated the existence of a heat-stable factor in synaptosomal cytosol that regulates the phosphorylation of a particular protein in neuronal membranes. This phosphorylation is both cAMP- and calcium-independent. This factor also appears to affect phosphorylation of cytosolic proteins. Since protein phosphorylation has been linked to release of neurotransmitter, phosphorylation system may play an important role in synaptic function. It is therfore of interest to define the factors affecting the steady or dynamic phosphorylation states of neuronal proteins.